WASHINGTON (Reuters) - Federal investigators wiretapped the phone lines of U.S. President Donald Trump's longtime lawyer Michael Cohen before the raid on his offices, hotel room and home, NBC News reported on Thursday, citing sources familiar with the legal proceedings.

The raids occurred in April as part of a federal criminal investigation in New York partly over a payment made to adult film star Stormy Daniels, who has alleged an affair with Trump a decade ago. Trump called her claims "false and extortionist" in a tweet on Thursday.

The New York investigation is an offshoot of the ongoing probe by U.S. Special Counsel Robert Mueller into Russian meddling in the 2016 election and whether the Trump campaign colluded with Moscow. Both Russia and Trump deny any wrongdoing.

It was not immediately clear when the warrant for surveillance was obtained or what evidence the Federal Bureau of Investigation had to support its request.

Rudy Giuliani, the former mayor of New York City turned Trump's personal attorney, told the Washington Post that the wiretaps were "not appropriate," according to a Twitter post by the Post reporter. Giuliani did not immediately return a call for comment from Reuters.

A spokesman for the Manhattan U.S. Attorney's office, which is handling the Cohen investigation, declined to comment. Cohen and a lawyer for him all did not immediately respond to requests for comment.